A double flywheel of the above kind is disclosed in the specification of U.S. Pat. No. 4,903,544 and the corresponding French published patent application No. FR 2 626 336A, in which the viscous damping means is arranged radially outwards of the resilient means, being mechanically interposed between the two inertia masses. The viscous damping means comprises a sealed cavity which is filled with a fluid, and is carried axially on a first one of the two inertia masses. The viscous damping means is dimensioned according to the application to which the flywheel is to be put, for the purpose of giving a predetermined degree of damping. It includes two closure members or covers, which are secured together by means of a spacer element in order to define the said cavity.
The cover members are fixed with respect to a damper plate, optionally with a clearance, at least to the extent that they rotate with the damper plate. The damper plate is arranged to come into engagement with the resilient means, which are operatively interposed between the two inertia masses. The damper plate may constitute the spacer means defining the cavity of the viscous damping means, or it may be separate from that cavity. In addition, the viscous damping means includes an outer carrier element which is provided with teeth directed radially inwardly (i.e. towards the axis of the assembly), and an inner carrier element having further teeth which are directed radially outwardly (i.e. away from the axis of the assembly). The teeth of the outer carrier element are arranged to alternate with those of the inner carrier element.
In that type of arrangement, a problem arises from the fact that the viscous damping means is arranged to be effective to apply damping forces over large relative angular displacements between the two inertia masses, especially on stopping and starting of the engine when the resonant frequency is passed. This viscous damper can however produce perturbations during normal operation, especially due to shear forces in the fluid between the teeth and the cover members.
In order to overcome this drawback, it might be conceived that the viscous damping means could be locked, for example with the aid of locking members which are responsive to the displacement between the two inertia masses. However, such an arrangement would call for considerable modification of the viscous damper.